In a scanning electron microscope, a subject is irradiated with a high voltage beam of electrons that sweeps across the subject in a raster that is analagous to the raster that an electron beam forms on the face of a cathode ray tube of a television set. In response to the primary electrons from the beam, the subject emits a relatively large number of secondary electrons that travel outwardly in a hemispherical pattern. An electron detector detects these electrons and ultimately produces a corresponding video signal of the point of high voltage impact. The video signal varies with the specimen-beam interaction. A variation in secondary electrons is due to topography, composition, crystallinity, and magnetic or electronic effects.
A cathode ray tube monitor has its electron beam sweep synchronized with the electron beam sweep of the scanning electron microscope and the video signal from the detector modulates the beam of the CRT monitor. Thus, the monitor displays an image of the subject.
Electron microscopes are capable of magnification on the order of 100,000 times. However, the electron microscope is a complex instrument and its performance can be degraded from various causes. Thus, it is a practice to begin an operation with a scanning electron microscope by viewing a standard test subject that has a recognizable appearance when the scanning electron microscope is operating properly. Such a test subject is called a resolution standard.
Using a resolution standard is somewhat analagous to testing the quality of a low power magnifying glass by viewing the fine details of the engraving on a dollar bill. With a good lens of appropriate magnification, fine details of the engraving can be clearly seen. If the dollar bill is out of focus or if the lens is of poor quality or if the surface of the lens needs cleaning, the image is correspondingly unclear and small details cannot be resolved. To consider this analogy further, the dollar bill functions well as a resolution standard because it has distinguishable features of a known approximate size. (A resolution standard is not used to calibrate an exact unit of measurement but it can be combined with a calibration standard). As an additional parallel, the resolution standard has a suitable pattern over a particular area so that it will ordinarily be possible to find a readily usable area for the test even though particular areas may become unusable. If the image of the resolution standard is unsatisfactory, this can be confidently attributed to the imaging system or to other instrument problems such as vacuum leaks, vibrations and the like. Alternatively, a defective resolution standard can be readily identified and replaced.
Dendritic aluminum tungstem has been proposed as a resolution standard for a scanning electron microscope. ("The 30th Annual Proceedings of Electron Microscope Society", Los Angeles, Calif., C. J. Arceneaux, Editor.) To a scanning electron microscope, the dendritic aluminum tungsten growth presents details on the order of 100 Angstroms. A variety of other devices have been proposed that similarly provide observable details in the range of resolution that is required for many subjects for which scanning electron microscope is used.